How can I detect if a view is actually the "preview" view? In ST2 you could check if the view would be in the list of open files (the preview would not be in that list), but in ST3 I see the preview shows up as a regular open file.

I thought "window.transient_view_in_group(group)" would do the trick, but I'm not getting very far. Maybe I'm misunderstanding. I'd much appreciate it if anyone could help me out with this or point me in the right direction.

In st3 there's a concept of semi-transient views. I think they were introduced in the interest of user-friendliness. You're problably looking at one of those, although I don't know how they manifest themselves through the API...